|Publication number||US7961664 B1|
|Application number||US 11/152,026|
|Publication date||Jun 14, 2011|
|Filing date||Jun 13, 2005|
|Priority date||Mar 27, 2004|
|Publication number||11152026, 152026, US 7961664 B1, US 7961664B1, US-B1-7961664, US7961664 B1, US7961664B1|
|Inventors||Kristofer S. J. Pister, Robert M. Shear|
|Original Assignee||Dust Networks, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (67), Non-Patent Citations (4), Referenced by (3), Classifications (10), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a continuation in part of co-pending U.S. patent application Ser. No. 10/960,709 entitled LOW-POWERED AUTONOMOUS NODE FOR MESH COMMUNICATION NETWORK filed Oct. 6, 2004, which is incorporated herein by reference for all purposes and which claims priority to U.S. Provisional Patent Application No. 60/557,148 entitled COMMUNICATIONS PROTOCOL FOR REMOTE SENSOR NETWORKS filed Mar. 27, 2004 which is incorporated herein by reference for all purposes.
U.S. patent application Ser. No. 10/960,709 entitled LOW-POWERED AUTONOMOUS NODE FOR MESH COMMUNICATION NETWORK filed Oct. 6, 2004 is a continuation in part of U.S. patent application Ser. No. 10/914,056 entitled DIGRAPH BASED MESH COMMUNICATION NETWORK filed Aug. 5, 2004, which is incorporated herein by reference for all purposes, and claims priority to U.S. Provisional Patent Application No. 60/557,148 entitled COMMUNICATIONS PROTOCOL FOR REMOTE SENSOR NETWORKS filed Mar. 27, 2004, which is incorporated herein by reference for all purposes.
Digraph networks are networks of wireless nodes linked together using directional communication links. Digraph networks can be large. In some situations, communication is only required within a small group of nodes within the network. For example, the information from one node is only required by one or a few other nodes in the network. In another example, the information is required in a short time frame by another node. A problem that arises in a digraph network is that information flows in the network are distributed. The distribution of the information flow helps the reliability of the network but also can be inefficient in that a greater number of nodes are required to handle information than require the information. Another problem that arises is that the distributed information flow is inefficient in the time it takes to traverse the network. Control systems that rely on the timely arrival of information from one node at another node have difficulty if there is excess delay in the arrival of the information. It would be useful to have efficient use of node communications and to be able to achieve required timely arrival of information between nodes.
Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings.
The invention can be implemented in numerous ways, including as a process, an apparatus, a system, a composition of matter, a computer readable medium such as a non-transitory computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention.
A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured.
Subnetwork communication for a digraph linked network is disclosed. The subnetwork is a set of nodes and links between nodes that are a part of the network. Subnetwork communication has communication between nodes that are within the subnetwork and uses nodes and links that are within the subnetwork. Subnetworks can offer long superframes which provide slow communication and short superframes which provide fast communication between nodes in the network.
Digraph networks employ intelligent nodes comprising a transmitter and receiver, a power source, input devices, sometimes output devices, and an intelligent controller, such as a programmable microprocessor controller with memory. In the past, networks, such as the interne, have been developed having configurations or networks for communication that are static, dynamic or a hybrid of static and dynamic. Power for these networks has been most often supplied via wires (the nodes are “plugged in”) or occasionally from batteries. As the size, power, and cost of the computation and communication requirements of these devices has decreased over time, battery powered wireless systems have gotten smaller and smaller and more prevalent. The limit to size scaling resulting from this trend to smaller and lower power wireless devices is in the millimeter size range, leading to predictions of “smart dust”. For this reason, the research community has adopted the name mote to refer to a small wireless sensor device. Mote is an old English word meaning a speck of dust.
A self-contained unit of communication information is called a packet. A packet has a header, a payload and an optional trailer (
In ATM systems, a cell is a channel-specific time period of fixed duration during which a unit of communication occurs between two fixed terminals without conflict. By comparison, as used herein, a slot refers to a time period during which a packet can be sent as well as acknowledged, and a cell refers to a particular slot and radio channel offset in a superframe (defined below). In conventional TDMA systems, such as defined by the DS-1 (T-1) standard, a frame is a period of time of defined and fixed duration. By contrast, a superframe is an arbitrary number of slots and thus can be of variable duration. A superframe is iterated each cycle, as hereinafter explained.
Communication between intelligent nodes occurs only at specific times and on specific channels. Each intelligent node in a network represents its connectivity to other intelligent nodes in the network as a collection of directed links on one or more digraphs. Each superframe repeats in a continuous sequence of cycles. Each link can be used for the transmission and optional acknowledgement of a single packet. Thus, in a given superframe, the available bandwidth from intelligent node A to intelligent node B (in packets per second) is the product of the number of links from A to B in the superframe (links per cycle) and the superframe rate (cycles per second). For example, if there were 1 link from intelligent node A to intelligent node B in superframe S, and superframe S consisted of 100 slots of duration 50 ms per slot, then the length of a single cycle of superframe S would be five seconds (100×0.05=5), and the superframe rate would be 0.2 cycles/second. With one available link per frame, intelligent node A would be able to send at most one packet to intelligent node B every five seconds. In the same superframe, intelligent node B might have ten links to intelligent node A, giving B ten times the bandwidth to A as A has to B. In a separate superframe F with 10 slots of length 50 ms, intelligent node A might have five links to intelligent node C, giving an available bandwidth of 10 packets per second (1 packet/link*5 links/cycle*2 cycles/second).
The ability to create multiple superframes of different lengths, and assign different numbers of links between intelligent nodes in each superframe provides flexibility to the network designer. This flexibility allows bandwidth, redundancy, latency, and many other network performance parameters to be traded off against power consumption.
There is a one to one correspondence between digraphs or networks and superframes. Digraphs are the abstract representation of a superframe, and they allow designers to look at and design collections of links and understand their function. Each link in a digraph is assigned a cell, that is, a particular time slot offset and channel offset, in the corresponding superframe. In each cycle of the superframe, these two offsets are used together with the cycle number to calculate the exact time and frequency on which the intelligent node is to turn on its radio.
Herein the concept of digraph-based packet transport is introduced. Digraph based packet transport is analogous to water flowing in a river delta with its meandering branches. If a number of intelligent entities each in an independent unpropelled watercraft were dropped all over the delta with no means of guidance except to choose a path at each fork, they would take a wide variety of paths, depending on flow and congestion. Eventually, all would arrive at the basin. Two that started far apart might end up close together, and two that started near each other might take completely different paths and arrive at different times.
In a packet communication network, a method and apparatus for packet switched transport is provided among intelligent nodes wherein the duty cycling of the intelligent nodes is minimized in order to maximize power life using a synchronization algorithm that assures all nodes are able to propagate information through the network without undue use of transmission and reception power. Frequency hopping time-division multiple access supports packet communication between intelligent nodes via assigned directed links, each link being assigned to a time-channel offset (cell) in a superframe, so that a link carrying a packet string between any two intelligent nodes is active only during its assigned time slot. The result is efficient use of spectrum and minimal expenditure of power. If multiple superframes are employed and all frequency slots are simultaneously operating in synchronicity, the spectrum has the potential for 100% data utilization, less guard band spectrum.
In a sensor network, the vertices of a graph, representing the topology of the network, are the sites of intelligent nodes, also designated “motes,” either physical or symbolic, which are capable of analyzing incoming traffic and sensory data and which can act upon the traffic, reroute traffic and originate information from the site. Directed links (101
Superframes repeat, and each repetition is called a communication cycle, or simply a cycle. All intelligent nodes in a defined network share the same synchronized view of the occurrences of the edges of slots. Intelligent nodes may participate in multiple graphs/superframes and therefore communicate with other intelligent nodes at very different rates and with different latencies. There may be no single superframe which has slots that contains the traffic of all intelligent nodes in the network.
All intelligent nodes within a network, whether digraph-based or tree or conventional mesh, have a shared sense of time, synchronized to within about one millisecond (see below). In operation (
The duration of the “acceptable header start time” depends on the accuracy of clock synchronization (in parts per million) among intelligent nodes, as well as the length of delays between exchanges of packets and acknowledgment packets. Taken together, these parameters relate network latency, battery life and the superframe rate, or “chattiness” of the network.
Time clocks drift with temperature. For a reasonably simple temperature compensation scheme, the intelligent nodes are expected to a shared time base that is off by no more than a few tens of parts per million (PPM). For a 100 second long superframe, that corresponds to a few milliseconds of error after one cycle of a superframe. If longer time periods for superframe length are desired (i.e. less communication chatter) then either the listening time must be increased, with corresponding power increase, or the clock drift must be reduced.
An example of channel (frequency) assignment of three links in the multi-digraph is shown in
Multiple cycles of a sample superframe, as shown in
A relatively a large number of links may be implemented. For an embodiment with 50 channels and a 30 ms slot length, there are over 1500 slot/channel pairs available per second, with room for guard band and guard times.
In any network type, each intelligent node must store its own collection of links. The information that an intelligent node must store to completely characterize a link is about 20 bytes long (superframe ID, slot, channel, partner, link type, etc.). This allows an intelligent node to store roughly 50 links/KB of RAM.
The representation and synchronization of time in a sensor network is essential if the network is synchronous and if energy is to be conserved. In a specific embodiment, all intelligent nodes are assumed to have a 32 kHz crystal oscillator, as typically found in a watch, as a time reference. Low-cost watch crystals are advantageous because of their low power consumption (sub-microwatt), but they are known to have substantial variation in frequency in both their manufacturing tolerance, as well as their temperature dependence. One simple representation of the actual oscillation frequency of a crystal oscillator is given by:
F osc =F nom·(1+alpha(T−T nom +T off)2+PPMoff+PPMdrift)
where Fnom, is 32,768 Hz, Tnom is 25 degrees C., and alpha is typically 0.0035+/−0.0005 PPM/K2, Toff is +/−5K, PPMoff is +/−20 PPM, and PPMdrift is +/−3 PPM in the first year.
Taken together, these offsets and temperature dependencies generate a worst case of up to 200 PPM difference in crystal oscillation rate between two intelligent nodes under industrial temperature conditions. Even at room temperature, the difference can be almost 50 PPM. This implies that, without compensation, two intelligent nodes which synchronize their clocks at time t0 could be off by as much as 12 ms one minute later, or 17 seconds after a day.
The majority of the error in clock rate is due to the quadratic dependence of frequency on temperature. The room-temperature frequency offset, PPMoff, as well as the quadratic coefficient, alpha, and the temperature peak offset, Toff, can all be measured at the time of manufacture, and a calibration table can be created which represents the difference between the actual frequency of the oscillator and the desired frequency of the oscillator. It is difficult to directly adjust the frequency of the crystal based on this knowledge, but it is relatively straightforward to adjust the digital counter which is incremented each cycle of the crystal.
The calibration can be performed as follows: A simple method is to use table-lookup mapping technique to relate temperature to the appropriate delay for an extra “tick” of the 32 kHz clock. For example, if calibration determines that, at 0 deg. C., the clock is slow by 50 PPM, then every 20,000 ticks (the reciprocal of 50 PPM) a one tick adjustment is added.
A block diagram of a circuit 150 that illustrate this approach to calibration is shown in
In addition to uptime, each intelligent node maintains a local standard time offset 160, which is an estimate of the difference between its internal clock (uptime 158) and a global network standard, herein Dust Standard Time (DST) 162. DST is zero at midnight on Jan. 1, 2003. The DST offset 160 value is subject to modification from a communication information corrector 161 that derives correction information from the MAC layer. The DST offset 160 value is added to uptime value to generate the intelligent node's best guess at the DST 162 value, with correction. The DST value is used to schedule network events, such as communication with other intelligent nodes and for sampling of sensors. If an intelligent node is a part of multiple networks or has multiple parents or gateways, the DST offset for each can be stored separately. Time synchronization across the network is achieved by exchanging timing information in every link.
Groups of crystals manifest a natural distribution above and below their rated frequency, and only those which are slow can be sped up to the nominal frequency, leaving the other crystals to run fast. One solution to this problem is to speed all crystals up to a speed that is faster than the nominal crystal frequency.
For a given digraph, or communication flow, there are many different superframe implementations.
In the example illustrated in 10D, Ch 1-S1 cell, Ch 1-S4 cell, Ch 0-S5 cell, and Ch 2-S6 cell contain a communication loop that is cycled through multiple times (two times in this case) allowing frequent communication within the subnetwork. The superframe is nine time slots overall in length. One communication loop is where a packet is transmitted by a first node (node B3) and received by a second node (node B2) and where a packet is also transmitted by the second node (node B2) and received by the first node (node B3).
In the example illustrated in
In the example illustrated in Figures F, G, and H, superframes D, E, and F all run concurrently. Superframe E has a long latency (corresponding to the 200 time slots) and can be used for maintaining time synchronization, sending data and health reports, and control information. Superframe D has a medium latency (corresponding to 30 time slots). In some embodiments, node B is a heating ventilation and air conditioning control point and node H is a temperature sensor where the 30 time slots corresponds to approximately a 1 second latency. Superframe F has a short latency (corresponding to 1 time slot). In some embodiments, node F is a light switch and node A is a light and the 1 time slot corresponds to approximately a 30 millisecond latency. In various embodiments, superframes D, E, and F run sequentially and the network switches between the superframes based either on triggering events or on predetermined times.
Superframe G contains N channels (represented by Ch 0, Ch 1, and Ch N) and 65 time slots (represented by S0, S1, S15, and S64). In Ch 0-S0 cell, node F sends to node E. In Ch N-S1 cell, node E sends to node A. In Ch 0-S15 cell, node F sends to node A. In Ch 1-S15 cell, node H sends to node E. In Ch N-S64 cell, node H sends to node F. Superframe H contains N channels (represented by Ch 0, Ch 1, and Ch N) and 200 time slots (represented by S0, S1, S60, S142, S143, and S199). In Ch 0-S0 cell, node F sends to node C. In Ch N-S0 cell, node A sends to node E. In Ch 1-S1, node H sends to node C. In Ch 0-S60 cell, node B sends to node G. In Ch N-S60 cell, node E sends to node C. In Ch 1-S142 cell, node H sends to node B. In Ch N-S142 cell, node C sends to node G. In Ch 1-S143 cell, node B sends to node G. In Ch 0-S199 cell, node A sends to node B. In Ch 1-S199 cell, node F sends to node E.
In the example illustrated in Figures I and J, superframes G and H run concurrently. Superframe H has a long latency (corresponding to the 200 time slots) and can be used for maintaining time synchronization, sending data and health reports, and control information. Superframe G has a medium latency (corresponding to 65 time slots). In some embodiments, node H is an industrial process sensor and node A is an industrial process controller. In various embodiments, superframes G and H run sequentially and the network switches between the superframes based either on triggering events or on predetermined times.
Superframe I contains N channels (represented by Ch 0, Ch 1, and Ch N) and 2 time slots (represented by S0 and S1). In Ch 0-S0 cell, node G sends to node E. In Ch 1-S0 cell, node C sends to node A. In Ch N-S1 cell, node E sends to node C. Superframe J contains N channels (represented by Ch 0, Ch 1, and Ch N) and 2 time slots (represented by S0 and S1). In Ch 0-S0 cell, node H sends to node B. In Ch 1-S0 cell, node C sends to node A. In Ch 0-S1 cell, node H sends to node C. In Ch N-S1 cell, node B sends to node A. Superframe K contains N channels (represented by Ch 0, Ch 1, and Ch N) and 200 time slots (represented by S0, S1, S60, S142, S143, and S199). In Ch 0-S0 cell, node B sends to node A. In Ch N-S0 cell, node F sends to node C. In Ch 0-S1, node G sends to node E. In Ch 1-S1 cell, node H sends to node B. In Ch N-S1 cell, node C sends to node A. In Ch 0-S60 cell, node B sends to node A. In Ch N-S142 cell, node E sends to node C. In Ch 0-S143 cell, node G sends to node B. In Ch 1-S143 cell, node H sends to node C. In Ch 1-S199 cell, node F sends to node E.
In the example illustrated in Figures K and L, superframes K runs all the time and superframes I and J run only occasionally. Superframe K has a long latency (corresponding to the 200 time slots) and can be used for maintaining time synchronization, sending data and health reports, and control information. Superframe I has a short latency (corresponding to 2 time slots) with one payload from node G delivered to node A every 2 cycle. Superframe J has a short latency (corresponding to 2 time slots) with one payload from node H to node A in 1 cycle. In this example, superframe K takes longer to cycle through than superframe I or superframe J. In some embodiments, node G is a microphone sending real-time compressed voice information and node H is a camera transferring an image.
On a transmit link, it decides if it has a packet to send (or a beacon signal) 301 and adds destination and time of transmission stamps 302 to send, either direct or via broadcast 303. If direct it sets a timeout and listens for acknowledgment 304, identifies positive or negative acknowledgments 305 and either deletes the packet 306 and/or updates the standard internal time 307.
Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive.
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|U.S. Classification||370/315, 370/389, 370/338|
|International Classification||H04B7/14, H04W4/00, H04L12/56|
|Cooperative Classification||H04W52/0219, H04W40/02, H04W84/18|
|Jul 26, 2005||AS||Assignment|
Effective date: 20050629
Owner name: DUST NETWORKS, INC., CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:PISTER, KRISTOFER S. J.;SHEAR, ROBERT M.;REEL/FRAME:016576/0251
|Nov 21, 2014||FPAY||Fee payment|
Year of fee payment: 4